β-nitro derivatives of germanium(IV) corrolates

Article abstract of DOI:10.1021/ic801421a

The reaction between germanium(IV) meso-triphenylcorrolates and nitrate salts affords the corresponding β-nitro substituted corroles in good yield. Chromatographic separation of the crude reaction mixtures enables isolation of a μ-oxo dimer along with the corresponding monomers bearing a hydroxy or methoxy group at an axial position of the germanium central metal ion. Depending on the reaction conditions, mono- or dinitro substituted complexes can be obtained. The substitution is highly regioselective in each case, giving only the 3-nitro or 3,17-dinitro derivative among the different possible isomers. Five of the synthesized complexes were examined by cyclic voltammetry and UV-visible spectroelectrochemistry in dichloromethane, and the dinitro μ-oxo dimer is structurally characterized.

Full text of DOI:10.1021/ic801421a

Inorg. Chem. 2008, 47, 11680-11687
ꢀ-Nitro Derivatives of Germanium(IV) Corrolates
Marco Mastroianni,† Weihua Zhu,‡ Manuela Stefanelli,† Sara Nardis, Frank R. Fronczek,§
Kevin M. Smith,*,§ Zhongping Ou,‡ Karl M. Kadish,*,‡ and Roberto Paolesse*,†
Dipartimento di Scienze e Tecnologie Chimiche, UniVersita` di Roma Tor Vergata, Via della
Ricerca Scientifica, 1, 00133 Rome, Italy, Department of Chemistry, UniVersity of Houston,
Houston, Texas 77204-5003, and Department of Chemistry, Louisiana State UniVersity,
Baton Rouge, Louisiana 70803
Received July 29, 2008
The reaction between germanium(IV) meso-triphenylcorrolates and nitrate salts affords the corresponding ꢀ-nitro
substituted corroles in good yield. Chromatographic separation of the crude reaction mixtures enables isolation of
a µ-oxo dimer along with the corresponding monomers bearing a hydroxy or methoxy group at an axial position
of the germanium central metal ion. Depending on the reaction conditions, mono- or dinitro substituted complexes
can be obtained. The substitution is highly regioselective in each case, giving only the 3-nitro or 3,17-dinitro derivative
among the different possible isomers. Five of the synthesized complexes were examined by cyclic voltammetry
and UV-visible spectroelectrochemistry in dichloromethane, and the dinitro µ-oxo dimer is structurally
characterized.
Introduction
The new millennium was an important turning point for
corroles, because it signaled the beginning of an impressive
increase in published literature on these compounds.1 From
this point of view, it is interesting to note that the corrole
macrocycle (Figure 1) is not a novelty in the porphyrinoid
field but was first reported in the early 1960s by Johnson
and Kay when they were attempting to find a synthetic route
to vitamin B12.2 In recent years, the development of facile
Figure 1. Molecular structure of corrole.
and efficient synthetic routes for the preparation of triaryl-
corroles from commercially available precursors has been a
driving force for the increased “popularity” of this macro-
cycle,1,3 in part because it made the corroles more accessible
for study by a wider number of research groups and in part
because of the intriguing chemical character of this macro-
cycle.
The coordination chemistry of corroles differs in several
respects from that of the more ubiquitous porphyrins.4 A
characteristic feature of the corrole is the contracted mac-
rocycle with its trianionic ligand character, an attribute that
induces a stabilization of the central metal ion in higher
formal oxidation states than in the case of the corresponding
porphyrins whose macrocycles are dianionic ligands.5 The
natural consequence of a higher central metal oxidation state
* Authors to whom correspondence should be addressed. E-mail:
roberto.paolesse@uniroma2.it (R.P.), kkadish@uh.edu (K.M.K.),
kmsmith@lsu.edu (K.M.S.).
†
Universita` di Roma Tor Vergata.
University of Houston.
Louisiana State University.
(4) (a) The Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard,
R., Eds.; Academic Press: San Diego, CA, 2000; Vol. 1-10. (b) The
Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.;
Academic Press: San Diego, CA, 2003; Vol. 11-20.
‡
§
(1) Aviv, I.; Gross, Z. Chem. Commun. 2007, 1987–1999.
(2) Johnson, A. W.; Kay, I. T. J. Chem. Soc. 1965, 1620–1629.
(3) (a) Gryko, D. T. Eur. J. Org. Chem. 2002, 1735. (b) Ghosh, A. Angew.
Chem., Int. Ed. 2004, 43, 1918. (c) Gryko, D. T.; Fox, J. P.; Goldberg,
D. P. J. Porphyrins Phthalocyanines 2004, 8, 1091. (d) Nardis, S.;
Monti, D.; Paolesse, R. Mini-ReV. Org. Chem. 2005, 2,546. (e)
Koszarna, B.; Gryko, D. T. J. Org. Chem. 2006, 71, 3707.
(5) (a) Erben, C.; Will, S.; Kadish, K. M. In The Porphyrin Handbook;
Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic Press: San
Diego, CA, 2000; Vol. 2, p 233. (b) Gross, Z. J. Biol. Inorg. Chem.
2001, 6, 733. (c) Bro¨ring, M.; Bre´gier, F.; Tejero, E. C.; Hell, C.;
Holthausen, M. C. Angew. Chem., Int. Ed. 2007, 46, 445–448, and
references therein.
11680 Inorganic Chemistry, Vol. 47, No. 24, 2008
10.1021/ic801421a CCC: $40.75  2008 American Chemical Society
Published on Web 11/08/2008

ꢀ-Nitro DeriWatiWes of Germanium(IV) Corrolates
Chart 1. Molecular Structures of the Synthesized Corroles
and a higher electron density in the corroles leads to the so-
called “noninnocent” character of the macrocycle as a ligand,
which has sometimes complicated the interpretation of its
coordination and electrochemical behavior.6 On the other
hand, this behavior is important for exploiting the applica-
tions of metallocorroles in several areas such as, for example,
catalysis or chemical sensing, where coordination interaction
is fundamental for the working mechanism of the organic
material.1,7 For all of these applications, it is also important
to develop synthetic routes for the functionalization of
corrole, where different substituents are introduced at the
peripheral positions of the molecular framework.3d,8 This is
of particular importance for the construction of more
elaborate molecular architectures and for fine-tuning the
properties of the macrocycle itself.
The lower molecular symmetry of corroles compared with
porphyrins makes their functionalization especially chal-
lenging, because it introduces the problem of regioisomer
formation.8 In this regard, corroles have generally shown
an unexpected regioselectivity,9 with pyrroles A and D (and
in particular positions 3- and 17-) being more reactive than
pyrroles B and C (see Figure 1); this fact has been
successfully exploited to obtain regioselectively substituted
corroles in good yields.3d
Our laboratories have been interested for some time in
elucidating the reactivity of corroles in different substitution
reactions, and we have generally used the free-base macro-
cycle as a starting material.3d,10 This choice was due both
to the lack of facile protocols for the demetalation of
metallocorroles and to the interesting reactivity shown by
the free-base form of the corrole in these reactions. However,
in some cases, the exploitation of a metal complex is
necessary to drive a particular reaction towards the expected
products. From this point of view, we have recently shown
that it is possible to demetalate corroles under controlled
conditions,11 and this opportunity may allow novel important
perspectives for the functionalization of the macrocycle,
perhaps reaching the versatility obtained in the case of
porphyrins.12
tions. For example, the Ge(IV) corrolates are quite stable
and can be used under a variety of substitution reaction
conditions; they are also diamagnetic, allowing for facile
characterization of their reaction products. Furthermore, we
have recently shown that these metallomacrocycles may be
isolated in pure form after bromination of the macrocycle at
the ꢀ-pyrrole positions.14 This is not a trivial result, because
bromination is a reaction which has been difficult to control
with corroles, and in the past, only the fully brominated
product has been isolated in good yield.15
In this paper, we have expanded our earlier studies of Ge(IV)
corrolates to include the nitration reaction, which is a funda-
mentally useful functionalization reaction of tetrapyrrole mac-
rocycles12 and of aromatic molecules in general. The synthe-
sized corroles are shown in Chart 1. Three of the compounds
containing a nitrated macrocycle are examined as to their
electrochemistry and spectroelectrochemistry in nonaqueous
media, and these data are compared to the redox behavior of a
related monomer and µ-oxo dimer not having the added NO2
groups. The oxidation and reduction potentials were measured
by cyclic voltammetry, and UV-visible spectra of the elec-
trogenerated species were characterized by thin-layer spectro-
electrochemistry. One compound, the dinitro µ-oxo dimer, is
also structurally characterized.
Among the known metallocorroles, derivatives of germa-
nium(IV)13 offer several features that make them advanta-
geous for exploiting various corrole functionalization reac-
(6) Walker, F. A.; Licoccia, S.; Paolesse, R. J. Inorg. Biochem. 2006,
100, 810–837, and references therein.
(7) (a) Barbe, J.-M.; Canard, G.; Brande`s, S.; Guilard, R. Angew. Chem.,
Int. Ed. 2005, 44, 3103. (b) Barbe, J.-M.; Canard, G.; Brande`s, S.;
Guilard, R. Chem.sEur. J. 2007, 13, 2118–2129. (c) He, C.-L.; Ren,
F.-L.; Zhang, X.-B.; Han, Z.-X. Talanta 2006, 70, 364–369. (d) Gatto,
E.; Malik, M. A.; Di Natale, C.; Paolesse, R.; D’Amico, A.; Lundstro¨m,
I; Filippini, D. Chem.sEur. J.. [Online] DOI: 10.1002/chem.200800590.
(e) Paolesse, R.; Mandoj, F.; Marini, A.; Di Natale, C. InEncyclopedia
of Nanoscience and Nanotechnology; Nalwa, H., Ed.; American
Science Publishers: Valencia, CA, 2004.
(8) Saltsman, I.; Mahammed, A.; Goldberg, I.; Tkachenko, E.; Botoshan-
sky, M.; Gross, Z. J. Am. Chem. Soc. 2002, 124, 7411.
(9) Gross, Z.; Mahammed, A. J. Porphyrins Phthalocyanines 2002, 6,
553–555.
(10) (a) Paolesse, R.; Jaquinod, L.; Senge, M. O.; Smith, K. M. J. Org.
Chem. 1997, 62, 6193. (b) Paolesse, R.; Nardis, S.; Venanzi, M.;
Mastroianni, M.; Russo, M.; Fronczek, F. R.; Vicente, M. G. H.
Chem.sEur. J. 2003, 9, 1192. (c) Paolesse, R.; Nardis, S.; Stefanelli,
M.; Fronczek, F. R.; Vicente, M. G. H. Angew. Chem., Int. Ed. 2005,
44, 3047.
(11) Mandoj, F.; Nardis, S.; Pomarico, G.; Paolesse, R. J. Porphyrins
Phthalocyanines 2008, 12, 19–26.
(12) Jaquinod, L. In The Porphyrin Handbook; Kadish, K. M., Smith, K. M.
Guilard, R., Eds.; Academic Press: San Diego, CA, 2000; Vol. 1, p
201.
(13) (a) Paolesse, R.; Licoccia, S.; Boschi, T. Inorg. Chim. Acta 1990, 178,
9. (b) Simkhovich, L.; Mahammed, A.; Goldberg, I.; Gross, Z.
Chem.sEur. J. 2001, 7, 1041.
(14) Nardis, S.; Mandoj, F.; Paolesse, R.; Fronczek, F. R.; Smith, K. M.;
Prodi, L.; Montalti, M.; Battistini, G. Eur. J. Inorg. Chem. 2007, 2345–
2352.
(15) (a) Paolesse, R.; Nardis, S.; Sagone, F.; Khoury, R. G. J. Org. Chem.
2001, 66, 550–556. (b) Mahammed, A.; Gray, H. B.; Meier-Callahan,
A. E.; Gross, Z. J. Am. Chem. Soc. 2003, 125, 1162–1163. (c)
Golubkov, G.; Bendix, J.; Gray, H. B.; Mahammed, A.; Goldberg, I.;
Di Bilio, A. J.; Gross, Z. Angew. Chem., Int. Ed. 2001, 40, 2132. (d)
Palmer, J. H.; Day, M. W.; Wilson, A. D.; Henling, L. M.; Gross, Z.;
Gray, H. B. J. Am. Chem. Soc. 2008, 130, 7786–7787.
Inorganic Chemistry, Vol. 47, No. 24, 2008 11681

Mastroianni et al.
grade III, 100 g); elution with CH2Cl2 afforded a first green fraction,
which was crystallized from CH2Cl2/hexane to give blue-green
crystals of [3-(NO2)TPCorrGe]2O (22 mg, 34% yield). A second
green fraction was obtained after eluting with CHCl3; crystallization
from CH2Cl2/hexane afforded 3-NO2TPCorrGe(OH) as dark crystals
(10 mg, 15% yield). [3-(NO2)TPCorrGe]2O was successively
washed with 4 M HCl, dried over Na2SO4, and then added to the
3-NO2TPCorrGe(OH) fraction dissolved in CH2Cl2; crystallization
from CH2Cl2/MeOH afforded red-green crystals of
3-NO2TPCorrGe(OCH3) (30 mg, 45% yield).
Experimental Section
Reagents and solvents (Sigma-Aldrich, Fluka, and Carlo Erba
Reagenti) for synthesis and purification of Ge(IV) corrolates were
of synthetic grade and used without further purification. Silica gel
60 (70-230 mesh) and neutral alumina (Brockmann grade III) were
used for chromatography.
1H NMR spectra were recorded on a Bruker AV300 (300 MHz)
spectrometer. Chemical shifts are given in parts per million relative
to tetramethylsilane (TMS). UV-vis spectra were measured on a
Cary 50 spectrophotometer; more precise measurements were
performed on a Perkin-Elmer λ18 spectrophotometer equipped with
a temperature-controlled cell holder. Mass spectra (FAB mode) were
recorded on a VGQuattro spectrometer in the positive-ion mode
using m-nitrobenzyl alcohol (NBA, Aldrich) as a matrix.
Cyclic voltammetry was carried out at 298 K using an EG&G
Princeton Applied Research (PAR) 173 potentiostat/galvanostat. A
homemade three-electrode cell was used for cyclic voltammetric
measurements and consisted of a platinum button or glassy carbon
working electrode, a platinum counter electrode, and a homemade
saturated calomel reference electrode (SCE). The SCE was separated
from the bulk of the solution by a fritted glass bridge of low porosity
which contained the solvent/supporting electrolyte mixture.
Thin-layer UV-visible spectroelectrochemical experiments were
performed with a homemade thin-layer cell which has a light
transparent platinum net working electrode. Potentials were applied
and monitored with an EG&G PAR Model 173 potentiostat. Time-
resolved UV-visible spectra were recorded with a Hewlett-Packard
Model 8453 diode array spectrophotometer. High-purity N2 from
Trigas was used to deoxygenate the solution and kept over the
solution during each electrochemical and spectroelectrochemical
experiment.
[3-(NO2)TPCorrGe]2O. Anal found for C74H44Ge2N10O5: C,
68.6; H, 3.5; N, 10.5. Calcd: C, 68.4; H, 3.4; N, 10.8%. UV-vis
1
(CH2Cl2): λmax, nm 422 (ε 125 300), 601 (35 500). H NMR (300
MHz, CDCl3): δ 9.31 (s, 2H, ꢀ-pyrr.), 8.52 (d, 2H, J ) 4.4 Hz,
ꢀ-pyrr.), 8.43 (d, 2H, J ) 4.8 Hz, ꢀ-pyrr.), 8.37 (d, 2H, J ) 4.8
Hz, ꢀ-pyrr.), 8.09 (overlapping dd, 4H, ꢀ-pyrr.), 8.02 (d, 2H, J )
4.8 Hz, ꢀ-pyrr.), 7.5-7.9 (m, 30H, phenyls). LRMS (FAB): m/z
1298 (M+).
3-NO2TPCorrGe(OH). Anal. found for C37H23GeN5O3: C, 67.6;
H, 3.2; N, 10.5. Calcd: C, 67.5; H, 3.5; N, 10.6%. UV-vis
1
(CH2Cl2): λmax, nm 426 (ε 73 900), 607 (22 800). H NMR (300
MHz, CDCl3): δ 9.71 (s, 1H, ꢀ-pyrr.), 9.25 (d, 1H, J ) 4.1 Hz,
ꢀ-pyrr.), 9.09 (d, 1H, J ) 4.8 Hz, ꢀ-pyrr.), 9.00 (d, 2H, J ) 4.6
Hz, ꢀ-pyrr), 8.75 (dd, 2H, ꢀ-pyrr.), 8.40-7.65 (m, 15H, phenyls),
-4.02 (br s, 1H, OH). LRMS (FAB): m/z 658 (M+).
3-NO2TPCorrGe(OCH3). Anal. found for C38H25GeN5O3: C,
67.6; H, 3.6; N, 10.5. Calcd: C, 67.9; H, 3.8; N, 10.4%. UV-vis
1
(CH2Cl2): λmax, nm 426 (ε 73 900), 607 (22 800). H NMR (300
MHz, CDCl3): δ 9.72 (s,1H, ꢀ-pyrr.), 9.24 (d, 1H, J ) 4.3 Hz,
ꢀ-pyrr.), 9.09 (d, 1H, J ) 4.8 Hz, ꢀ-pyrr.), 9.00 (d, 2H, J ) 4.4
Hz, ꢀ-pyrr), 8.76 (d, 1H, J ) 5.2 Hz, ꢀ-pyrr.), 8.75 (d, 1H, J ) 5.1
Hz, ꢀ-pyrr.), 7.6-8.4 (m, 15H, phenyls), -0.62 (s, 3H, OCH3).
LRMS (FAB): m/z 672 (M+).
Dichloromethane (CH2Cl2) and pyridine (py) were obtained from
Aldrich Co. and were used as received for electrochemistry and
spectroelectrochemistry experiments. Tetra-n-butylammonium per-
chlorate (TBAP) was purchased from Sigma Chemical or Fluka
Chemika Co., recrystallized from ethyl alcohol, and dried under a
vacuum at 40 °C for at least one week prior to use.
Method B. TPCorrGe(OCH3) (90 mg, 0.144 mmol) and NaNO3
(32 mg, 0.36 mmol) were dissolved in 15 mL of acetic anhydride
and stirred at 45 °C, and then 6 mL of acetic acid was added. The
solution color became deep green, and after 30 min, TLC showed
no more starting material in the reaction mixture. The solvent was
evaporated and the residue dissolved in CH2Cl2 and washed with
saturated aqueous NaHCO3; the organic phase was concentrated
and chromatographed on a neutral alumina (Brockmann grade III)
column (100 g); a first green fraction was obtained using CH2Cl2
as the eluant, and the residue was crystallized from CH2Cl2/hexane
to give green crystals of [3,17-(NO2)2TPCorrGe]2O (12 mg, 12%
yield); elution with CHCl3 afforded a second green band that was
crystallized from CH2Cl2/MeOH to give directly 3,17-
(NO2)2TPCorrGe(OCH3) (36 mg, 35% yield).
X-Ray Crystal Data. Diffraction data were collected on a Nonius
KappaCCD diffractometer equipped with graphite-monochromated
Mo KR radiation (λ ) 0.71073 Å) and an Oxford Cryostream low-
temperature device. Crystal data for [3,17-(NO2)2TPCorrGe]2O:
C74H42N12O9Ge2 ·2CHCl3, monoclinic, space group P21/n, a )
18.634(2), b ) 16.1855(15), c ) 23.728(4) Å, ꢀ ) 94.528(6)°, V
) 7134.0(15) Å3, Z ) 4, Dcalcd ) 1.515 g cm-3, µ ) 1.136 mm-1
,
T ) 90 K, 66 866 reflections collected with θmax < 26.4°, 14 509
independent reflections (Rint ) 0.046) which were used in all of
the calculations. The chloroform molecules are disordered, and their
contribution to the structure factors was removed using SQUEEZE
[Spek, A. L. J. Appl. Crystallogr. 2003, 36, 7-13]. Final residuals
(for 874 parameters) were R1 [I >2σ(I)] ) 0.049 and wR2 (all data)
) 0.134, CCDC 692211.
[3,17-(NO2)2TPCorrGe]2O. Anal. found for C74H42Ge2N12O9:
C, 63.9; H, 3.1; N, 11.9. Calcd: C, 64.0; H, 3.0; N, 12.1%. UV-vis:
1
λ
max, nm (CH2Cl2): 422 (ε 126 900), 601 (41 500). H NMR (300
Synthesis. TPCorrH3 and TPCorrGe(OCH3) were prepared as
MHz, CDCl3): δ 9.20 (s, 2H, ꢀ-pyrr.), 8.48 (d, 2H, J ) 4.9 Hz,
ꢀ-pyrr.), 8.07 (d, 2H, J ) 4.9 Hz, ꢀ-pyrr.), 7.4-8.0 (m, 15H,
phenyls). LRMS (FAB): m/z 1388 (M+).
3,17-(NO2)2TPCorrGe(OCH3). Anal. found for C38H24GeN6O5:
C, 63.2; H, 3.5; N, 11.5. Calcd: C, 63.6; H, 3.4; N, 11.7%. UV-vis:
previously reported.3,14
Nitration of TPCorrGe(OCH3). Method A. A solution of
TPCorrGe(OCH3) (62 mg, 0.1 mmol) in CH2Cl2 (35 mL) was
treated with 3.5 mL of a solution of LiNO3 (80 mg) in AcOH (4
mL) and Ac2O (2.3 mL), and the mixture was heated at reflux.
The color of the solution turned from purple to green, and after 5
min, a TLC examination revealed no starting material; the mixture
was diluted with CH2Cl2 (35 mL), washed three times with water,
dried over Na2SO4, and the solvent then evaporated. The residue
was chromatographed on a neutral alumina column (Brockmann
λmax, nm (CH2Cl2): 428 (ε 43 700), 449 (sh., 38 200), 630 (21 100).
1H NMR (300 MHz, CDCl3): δ 9.57 (s, 2H, ꢀ-pyrr.), 8.97 (d, 2H,
J ) 5.0 Hz, ꢀ-pyrr.), 8.67 (d, 2H, J ) 5.0 Hz, ꢀ-pyrr.), 7.7-8.2
(m, 15H, phenyls), -0.63(s, 3H, OCH3). LRMS (FAB): m/z 717
(M+).
11682 Inorganic Chemistry, Vol. 47, No. 24, 2008

ꢀ-Nitro DeriWatiWes of Germanium(IV) Corrolates
Results and Discussion
The regioselectivity of the reaction is worth noting because
only one isomer is obtained from the ꢀ-substitution reaction.
According to the high reactivity shown at the 3- position in
analogous triarylcorrole functionalizations3d and to the
spectral characterization, in particular 1H NMR characteriza-
tion that supports the site of this substitution at the 3- position
of the macrocycle,18 these Ge(IV) derivatives can be confidently
identified as the 3-nitro substituted species, as further confirmed
by the results described later in the manuscript. Unfortunately,
all attempts to obtain single crystals of these derivatives failed,
thereby thwarting their unambiguous characterization. It should
be noted that the regioselectivity observed is the same reported
for the nitration reaction of the Ga complex of the 5,10,15-tris-
(pentafluorophenyl)corrole.8
Synthesis and Characterization. Preparation of the
Ge(IV) corrolates was performed by reacting the free-base
macrocycle with GeCl4 in refluxing DMF.14 It was earlier
reported that this reaction affords two complexes, the µ-oxo
dimer (TPCorrGe)2O and the corresponding monomer,
TPCorrGe(OH), both resulting from the facile hydrolysis of
the Ge-Cl bond on an initial TPPCorrGeCl intermediate.14
The monohydroxide and µ-oxo dimer species were both
converted into the corresponding methoxide complex,
TPCorrGe(OCH3), which was previously used as a model
compound for the bromination reaction.14 (TPCorrGe)2O was
first treated with dilute HCl and then crystallized from
CH2Cl2/CH3OH, while the direct crystallization of
TPCorrGe(OH) was sufficient to afford TPCorrGe(OCH3)
in quantitative yield.
The successful nitration of free-base corroles using AgNO2
as a nitrating reagent in CH3CN was recently reported.16
Following this promising result, we attempted the same
nitrating system in the case of TPCorrGe(OCH3), but this
reaction led mostly to the formation of ring-opened species,
with only traces of the desired macrocyclic substituted
product(s) being obtained. Open-chain products were also
earlier observed when reacting the pentafluorophenyl deriva-
tive TF5PCorrH3 with AgNO2.
To explore the possibility of obtaining poly substitution
products, we elected to use reaction conditions moderately
more severe than those exploited earlier, using a molar excess
of NaNO3 in Ac2O/AcOH. The choice was successful, and
a chromatographic separation of the crude mixture in this
case afforded two main products. The first band showed
traces of a compound that spectral characterization identified
as being identical with [3-(NO2)TPCorrGe]2O. The subse-
quent fraction afforded again a µ-oxo dimer of a disubstituted
Ge corrole derivative, as shown by FAB mass spectrometry.
1
The H NMR spectrum indicated a symmetrical pattern for
the substitution, indicating formation of the 3,17-dinitro
µ-oxo dimer. Single crystals of this compound were obtained
by slow diffusion of hexane into a diluted dichloromethane
solution, and an X-ray crystal structure determination
unambiguously confirmed the compound to be [3,17-
(NO2)2TPCorrGe]2O. Its structure is shown in Figure 2.
In the µ-oxo dimer, Ge1 lies 0.483(2) and Ge2 lies
0.398(2) Å out of their basal N4 planes, toward the interior
of the molecule. Ge2 has a long intermolecular distance of
2.921(2) Å to an O atom of a nitro group trans to the bridging
O, while Ge1 has no such axial interaction to either other
Ge dimers or solvent molecules.
The above failure to get the desired nitrated product led us
to explore alternative approaches, and we decided to use the
mild nitrating system, LiNO3/Ac2O/AcOH in CH2Cl2, which
was reported to be successful in the nitration of nonaromatic
Ni corrole complexes.17 Although in our previous work
electrophilic nitrating reagents did not give satisfying results
in terms of functionalizing the free-base corrole (due to
decomposition of the starting material16), both the mild condi-
tions and the presence of a corrole metal complex were
promising for the fate of the reaction. In this case, the reaction
was successful, and after 5 min, no starting material remained
in the crude mixture, as evidenced by TLC analysis.
The Ge-N distances in the five-membered chelate rings
are in the range 1.919(3)-1.929(3) Å, longer than the others,
being 1.902(3)-1.914(3) Å; Ge-O1 distances are 1.735(2)
Å for Ge1 and 1.738(2) Å for Ge2, and the Ge-O-Ge angle
is distinctly nonlinear, being 157.61(15)°. This causes the
two N4 planes to form a dihedral angle of 18.10(8)°. The
distance between the Ge atoms is 3.834(1) Å, and the
distance between the two N4 centers is 4.220 Å. Overall,
the dimeric molecule has approximate C2 symmetry, with
the two corrole rings twisted with respect to each other by
61.5° about the Ge · · · Ge vector. The individual corroles are
nonplanar, distorted somewhat into saddle conformations.
The 23 atoms of the corrole ring coordinating Ge1 exhibit a
mean deviation of 0.102 Å from coplanarity, with a
maximum of 0.238(3) Å. For the corrole coordinating Ge2,
these values are 0.120 and 0.291(3) Å.
The chromatographic separation performed on neutral
alumina afforded two main products. Spectral characteriza-
tion of the first fraction indicated that the µ-oxo dimer of a
monosubstituted Ge derivatives had been formed; FAB mass
spectrometry showed a molecular peak corresponding to a
1
the µ-oxo Ge(IV) complex, and the H NMR spectrum
showed a singlet at 9.31 ppm, indicating substitution at a
ꢀ-pyrrolic position of the macrocycle. The second fraction
was identified as the corresponding monosubstituted mono-
mer bearing a hydroxyl group at the Ge(IV) axial position.
Treatment of the µ-oxo dimer with dilute HCl also led to a
mixture of monomeric corroles, one with a Cl- axial ligand
and the other with bound OH-. This fraction was combined
with the hydroxide derivative directly obtained from the
reaction and crystallized from CH2Cl2/CH3OH to give the
corresponding methoxy corrole in 45% yield.
The structure of [3,17-(NO2)2TPCorrGe]2O is quite similar
to that of the corresponding compound lacking the nitro
groups.14 In that structure, the Ge-N distances are in the
(16) Stefanelli, M.; Mastroianni, M.; Nardis, S.; Licoccia, S.; Fronczek,
F. R.; Smith, K. M.; Zhu, W.; Ou, Z.; Kadish, K. M.; Paolesse, R.
Inorg. Chem. 2007, 46, 10791–10799.
(17) Ruppert, R.; Jeandon, C.; Callot, H. J. J. Org. Chem. 2008, 73, 694–
700.
(18) Balazs, Y. S.; Saltsman, I.; Mahammed, A.; Tkachenko, E.; Golubkov,
G.; Levine, J.; Gross, Z. Magn. Reson. Chem. 2004, 42, 624–635.
Inorganic Chemistry, Vol. 47, No. 24, 2008 11683

Mastroianni et al.
Figure 2. Molecular structure of [3,17-(NO2)2TPCorrGe]2O, with 50% ellipsoids. H atoms are not shown.
range 1.881(9)-1.930(9) Å, and the Ge-O distances are in
the range 1.718(11)-1.773(13) Å in two independent
molecules. Dihedral angles between the N4 planes in that
structure are 19.4(6) and 19.9(5)°, comparable to those in
[3,17-(NO2)2TPCorrGe]2O. However, its Ge-O-Ge angles,
147.3(5) and 149.0(4)°, are significantly smaller than in the
current structure, leading to smaller Ge · · · Ge distances,
3.349(2) and 3.359(2) Å.
It should be noted that the additional fraction in the
reaction mixture was the disubstituted monomer 3,17-
(NO2)2TPCorrGe(OH), which was directly converted into
the corresponding methoxy derivative, 3,17-(NO2)2-
TPCorrGe(OCH3), by crystallization from CH2Cl2/CH3OH.
The substitution is highly regioselective, and only the 3,17-
disubstituted isomer was isolated, the total yield of the
reaction being around 50%, thus indicating that the starting
complex is sufficiently stable under the given reaction
conditions. Also in this case, the 3,17- substitution pattern
observed is identical to that reported in the case of the Ga
complex of 5,10,15-tris(pentafluorophenyl)corrole,8 confirm-
Figure 3. UV-visible spectra of the mono-Ge(IV) corrolates.
ing this corrole regioselectivity in the nitration reactions.
The UV-vis spectra of the mono- and dinitrosubstituted
complexes in dichloromethane are shown in Figure 3. The
compounds all exhibit a Soret band at around 425 nm and a
broad Q-band close to 600 nm. Introduction of nitro
substituents at the peripheral positions of the macrocycle of
the triphenylcorrole induce significant variations in the
spectral features of the starting TPCorrGeOCH3, as illustrated
in the figure. Upon substitution, both the Soret and Q
absorption bands are red-shifted and considerably broadened.
There is also a significant increase of the relative Q-band
intensity. The addition of a second nitro group does not
induce similar changes in the absorption spectra. As already
observed in the case of unsubstituted Ge(IV) triphenylcor-
rolates,14 [3-(NO2)TPCorrGe]2O and [3,17-(NO2)2-
TPCorrGe]2O (Figure 4) show Soret bands which are higher
in intensity than those of the corresponding monomer, but
the corresponding shift to shorter wavelengths is in this
case less than that observed for the unsubstituted species.
The broadening of the Q bands, on the other hand, is not
accompanied by similar band shifts. Both features seem
to indicate a smaller degree of interaction among the two
macrocycles in the µ-oxo dimer. Both [3-(NO2)TP-
CorrGe]2O and [3,17-(NO2)2TPCorrGe]2O are unstable
upon dilution, and at concentrations lower than 10-6 M,
their spectra slowly revert to those of the corresponding
monomers.
Electrochemistry. Figures 5 and 6 and Figure S1 (Sup-
porting Information) illustrate cyclic voltammograms of the
five investigated Ge(IV) corrolates, while Table 1 sum-
marizes redox potentials of these compounds in CH2Cl2
containing 0.1 M TBAP and also includes the data for three
related compounds of Ge(IV), one a derivative of a Ge
complex of the 5,10,15-tris(pentafluorophenyl)corrole and the
other two of Ge(IV) tetraphenylporphyrin derivatives. Two
one-electron reductions, located at E1/2 ) -1.36 V and Epc
) -1.95 V and two one-electron oxidations, at E1/2 ) 0.79
and 1.22 V, are observed for TPCorrGe(OCH3) in CH2Cl2.
In contrast, up to three reductions and three oxidations can
11684 Inorganic Chemistry, Vol. 47, No. 24, 2008

ꢀ-Nitro DeriWatiWes of Germanium(IV) Corrolates
Figure 4. UV-visible spectra of the bis-Ge(IV) corrolates.
Figure 6. Cyclic voltammograms of mono-Ge corrolates and bis-Ge(IV)
corrolates in CH2Cl2 containing 0.1 M TBAP.
is observed for the investigated Ge(IV) corrolates; the
redox potentials of the compounds are all positively shifted
when one or two nitro groups are introduced onto the
ꢀ-pyrrole position of the macrocycle, and the reductions
are more effected than the oxidations by nitration. For
example, in the case of mononitration, the first reduction
is positively shifted by 440 mV, from E1/2 ) -1.36 V for
TPCorrGe(OCH3)
NO2)TPCorrGe(OCH3). At the same time, the first oxida-
tion is shifted by 190 mV, from 0.79 for
to
-0.92
V
for
(3-
V
TPCorrGe(OCH3) to 0.98 V for (3-NO2)TPCorrGe(OCH3)
(see Figure 5).
The first reduction and first oxidation of the monomeric
dinitro corrole, (3,17-NO2)2TPCorrGe(OCH3), are located at
E1/2 ) -0.63 and 1.13 V, respectively, in CH2Cl2 and 0.1
M TBAP. Compared with (3-NO2)TPCorrGe(OCH3), the E1/2
of the 3,17-dinitro derivative is only shifted by 290 mV for
reduction and 150 mV for oxidation (Figure 5). Thus, the
addition of a second nitro group to the macrocycle decreases
the effect of nitration on the redox potentials under the same
experimental conditions.
The potential difference between the first reduction and
first oxidation of TPCorrGe(OCH3) in CH2Cl2 and 0.1 M
TBAP (the HOMO-LUMO gap) is 2.15 V for (Table 1 and
Figure S1, Supporting Information), a separation within the
range of values previously reported for other metallocor-
Figure 5. Cyclic voltammograms showing the first reduction and first
oxidation of mono-Ge(IV) corrolates with zero, one, or two NO2 groups at
the ꢀ position(s) of the macrocycle. Measuring was made in CH2Cl2
containing 0.1 M TBAP.
be observed for the 3-nitro and 3,17-dinitro derivatives under
the same solution conditions (Table 1 and Figure S1,
Supporting Information).
The nitro functionality is a strong electron-withdrawing
group that shows a strong electronic interaction with the
porphyrin macrocycle and can affect both the HOMO and
LUMO energy levels.19 Generally, the addition of a single
nitro substituent on the ꢀ-pyrrole position of a metallo-
tetraphenylporphyrin such as (TPP)M, where M ) 2H,
Cu, Mg, or Zn, will lead to a 330-380 mV positive shift
in E1/2 for the first reduction but only a 100-110 mV
positive shift in the first oxidation as compared to the same
compound without the nitro groups.20-22 A similar trend
(20) Binstead, R. A.; Crossley, M. J.; Hush, N. S. Inorg. Chem. 1991, 30,
1259–1264.
(21) Giraudeau, A.; Callot, H. J.; Gross, M. Inorg. Chem. 1979, 18, 201–
206.
(22) Giraudeau, A.; Callot, H. J.; Jordan, J.; Ezhar, I.; Gross, M. J. Am.
Chem. Soc. 1979, 101, 3857–3862.
(19) Kadish, K. M.; Van Caemelbecke, E.; Royal, G. In The Porphyrin
Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic
Press: Amsterdam, 2000; Vol. 8, Chapter 55, pp 1-114.
Inorganic Chemistry, Vol. 47, No. 24, 2008 11685

Mastroianni et al.
Table 1. Half -Wave Potential (V vs SCE) of Ge(IV) Corroles in CH2Cl2, 0.1 M TBAP
oxidation
reduction
second
a
∆E1/2
compound
third
1.60
second
first
0.79
0.98
1.13
0.89, 0.66
1.15c
1.13
1.48
1.15
first
third
(V)
TPCorrGe(OCH3)
1.22
1.32
1.46
-1.36
-1.95b
2.15
1.90
1.76
2.17
1.78
2.13
2.17
2.24
(3-NO2)TPCorrGe(OCH3)
(3,17-NO2)2TPCorrGe(OCH3)
(TPCorrGe)2O
-0.92
-1.18
-0.63
-0.96
-1.58
-1.57
1.36, 1.24
-1.51c
-0.63, -0.79
-0.99
-1.71c
[(3,17-NO2)2TPCorrGe]2O
1.51c
-0.96, -1.24
TF5PCorrGe(OH)d
e
(TPP)Ge(ClO4)2
(TPP)Ge(OH)2
-0.69
-1.09
-1.56
-1.56
1.46b
e
a The HOMO-LUMO gap (potential difference between the first oxidation and first reduction). b Irreversible potential peak at a scan rate of 0.1 V/s.
c Two overlapping one-electron processes. d Data taken from ref 13b, TF5PCorr ) 5,10,15-tris(pentafluorophenyl)corrole. e In PhCN, data taken from ref 24.
roles.12,23 It is also similar to the HOMO-LUMO gap of
the Ge(IV) porphyrins (TPP)Ge(ClO4)2 and (TPP)Ge(OH)2
where ∆E1/2 ) 2.17 and 2.24 V, respectively24 (see Table
1).
Upon nitration, the HOMO-LUMO gap decreases to 1.90
V for (3-NO2)TPCorrGe(OCH3) and 1.76 V for (3,17-
NO2)2TPCorrGe(OCH3), respectively (see Table 1 and Figure
S1, Supporting Information).
The µ-oxo dimer, (TPCorrGe)2O, undergoes two reversible
reductions and four reversible one-electron oxidations within
the potential range of dichloromethane (Figure 6b). Each
reduction process involves two overlapping one-electron
additions, and a total of four electrons are added to the two
corrole macrocycles of the dimer upon scanning the potential
from 0.0 to -2.0 V versus SCE in CH2Cl2 and 0.1 M TBAP.
In contrast to the reduction, the first one-electron oxidation
does not occur at the same potential for each macrocycle of
the µ-oxo dimer, thus indicating an interaction between two
equivalent redox active sites on the molecule.19 The half-
wave potentials are located at E1/2 ) 0.66 and 0.89 V,
resulting in a 0.23 V potential separation between the two
Figure 7. UV-vis spectral changes of TPCorrGe(OCH3) during reduction
processes, the first of which gives a µ-oxo dimer with one
singly oxidized macrocycle and the second a dimer where
both macrocycles are oxidized by one electron. Stepwise
oxidations have also been reported for µ-oxo iron(III)
porphyrinates25 and porphycenates26 where the ∆E1/2 values
between each process range from 0.19 to 0.26 mV. The
second oxidation of each macrocycle in (TPCorrGe)2O also
occurs at different half-wave potentials, being 1.24 and 1.36
V as seen in Figure 6b. These results indicate that an
interaction also exists between the two singly oxidized
corrole macrocycles of the µ-oxo dimer.
Compared with (TPCorrGe)2O, different electrochemical
behavior is seen for the dinitro µ-oxo dimer under the same
solution conditions. Both the first and second oxidations of
[(3,17-NO2)2TPCorrGe]2O involve two overlapping one-
electron transfer steps which occur at the same potential for
each macrocycle. This is not the case for the first two
reductions of the compound, which occur in well-separated
at a controlled potential of (a) -1.65 and (b) -2.00 V in CH2Cl2 containing
0.1 M TBAP.
one-electron steps (see Figure 6d). This is consistent with
an interaction between the two neutral and singly reduced
corrole rings of the 3,17-dinitro µ-oxo Ge(IV) dimer, but
not between the two oxidized macrocycles, which are
equivalent and noninteracting under the given solution
conditions.
Spectroelectrochemistry. Thin-layer UV-visible spec-
troelectrochemistry was carried out under the same solution
conditions as for the cyclic voltammetry measurements.
Changes in the UV-visible spectra upon reduction or
oxidation of the different corroles are shown in Figures 7-9
and Figures S2-S4, Supporting Information).
Upon the first controlled potential reduction of
TPCorrGe(OCH3) at -1.65 V, the Soret band at 413 nm
and the visible bands at 560 and 589 nm decrease in intensity
while two low-intensity Soret bands and a broad visible band
grow in at 424, 443, and 639 nm (Figure 7a). The intensity
of the newly formed bands then decreases upon the second
controlled potential reduction at -2.00 V (Figure 7b). The
stepwise reduction products are assigned as a π-anion radical
and dianion. The spectral changes obtained during the first
and second oxidations are illustrated in Figure 8. The Soret
and visible bands decrease in intensity, but no new bands in
(23) Kadish, K. M.; Ou, Z.; Adamian, V. A.; Guilard, R.; Gros, C. P.;
Erben, C.; Will, S.; Vogel, E. Inorg. Chem. 2000, 39, 5675–5682.
(24) Kadish, K. M.; Xu, Q. Y.; Barbe, J. -M.; Anderson, J. E.; Wang, E.;
Guilard, R. Inorg. Chem. 1988, 27, 691–696.
(25) Kadish, K. M.; Autret, M.; Ou, Z.; Tagliatesta, P.; Boschi, T.; Fares,
V. Inorg. Chem. 1997, 36, 204–207.
(26) Kadish, K. M.; Boulas, P.; D’Souza, F.; Aukauloo, M. A.; Guilard,
R.; Lausmann, M.; Vogel, E. Inorg. Chem. 1994, 33, 471–476.
11686 Inorganic Chemistry, Vol. 47, No. 24, 2008

ꢀ-Nitro DeriWatiWes of Germanium(IV) Corrolates
lead to a stepwise formation of [TPCorr+•Ge-O-GeTPCorr]+
and [TPCorr+•Ge-O-GeTPCorr+•]2+ in the thin-layer cell.
The initial spectrum of the 3-nitro and 3,17-dinitro
derivatives before oxidation or reduction differs from that
of TPCorrGe(OCH3) in CH2Cl2 containing 0.1 M TBAP. The
Soret and visible bands of 3-nitro and 3,17-dinitro derivatives
are both broad (Figures 9 and Figure S4, Supporting
Information). Due to the electron-withdrawing effect of the
nitro group, the Soret band is shifted from 413 nm for
TPCorrGe(OCH3) to 428 nm for (3-NO2)TPCorrGe(OCH3)
and then to 432 nm for (3,17-NO2)2TPCorrGe(OCH3). The
Q-band of the compound is shifted from 589 in the absence
of a nitro group to 607 nm for the mononitro corrole and
then to 628 nm upon the addition of a second nitro group to
give the dinitro corrole.
The Soret and visible bands decrease in intensity while a
broad visible band between 650 and 900 nm grows in upon
the first reduction or first oxidation of the mono- and dinitro
derivatives in CH2Cl2 containing 0.1 M TBAP (Figures 9
and Figure S4, Supporting Information). This indicates that
π-anion radicals and π cations are formed during the first
reduction and first oxidation of these compounds, respec-
tively.
Figure 8. UV-vis spectral changes of TPCorrGe(OCH3) during oxidation
at a controlled potential of (a) 1.00 and (b) 1.40 V in CH2Cl2 and 0.1 M
TBAP.
Conclusions
We report here the preparation of ꢀ-nitro substituted
Ge(IV) triphenylcorrolates, obtained by reaction of the
starting corrole complex with nitrate salts/Ac2O/AcOH. This
is the first example of ꢀ-nitration of triphenylcorrole deriva-
tives carried out with electrophilic reagents. Depending on
the reaction conditions, it is possible to obtain mono- or bis-
nitro Ge(IV) corrolates; in all cases, the reaction proceeds
with high selectivity for substitution and only the 3- and 3,17-
derivatives are obtained among the huge number of possible
regioisomers. The ꢀ-nitro substituted Ge(IV) triphenylcor-
rolates are obtained from the substitution reaction as a
mixture of the µ-oxo dimer and the corresponding hydroxide
monomer. These species can be easily converted into the
corresponding methoxide complex.
This article presents also the first study on electrochemistry
and spectroelectrochemistry of the Ge(IV) corrolate com-
plexes, elucidating the site of electron transfer and the
influence of the nitro group. These ꢀ-nitro substituted
complexes are important intermediates, and we are currently
exploring these compounds for the preparation of more
elaborated species based on corroles.
Figure 9. UV-vis spectral changes of (3-NO2)TPCorrGe(OCH3) during
(a) reduction at -1.00 V and (b) oxidation at 1.10 V in CH2Cl2 and 0.1 M
TBAP.
the visible region are seen upon the first oxidation at 1.00 V
(Figure 8a). Similar spectra have been assigned to corrole
cation radicals,27 and a π-cation radical is assigned in the
present study. The spectra of the doubly oxidized species
are shown in Figure 8b and assigned to a dication.
Similar spectral changes are observed upon controlled
potential reduction and oxidation of (TPCorrGe)2O (Fig-
ures S2 and S3, Supporting Information). The reduction
at -1.65 V occurs at both macrocycles simultaneously
and leads to [TPCorr-•Ge-O-GeTPCorr-•]2- as the
spectrally detected product. The first two one-electron
oxidations of the starting compound at 0.80 and 1.00 V
Acknowledgment. The support of the Robert A. Welch
Foundation (K.M.K., Grant E-680) is gratefully acknowl-
edged.
Supporting Information Available: Cyclic voltammograms,
UV-visible spectroelectrochemical data, and a view of the mo-
lecular structure of [3,17-(NO2)2TPCorrGe]2O along the Ge · · ·Ge
vector. CIF file of [3,17-(NO2)2TPCorrGe]2O. This material is
available free of charge via the Internet at http://pubs.acs.org.
(27) Bendix, J; Dmochowski, I. J.; Gray, H. B.; Mahammed, A.; Simkhovich,
L.; Gross, Z. Angew. Chem., Int. Ed. 2000, 39, 4048–4051.
IC801421A
Inorganic Chemistry, Vol. 47, No. 24, 2008 11687